Field of the Invention
Ion implantation is currently taking a significant place in the process used for the manufacture of semiconductor devices. It is a powerful tool for introduction of significant useful impurities into semiconductor bodies as well as being utilized for other purposes such as surface tailoring for resistivity and threshold voltage control and improvement and for various other purposes well known to those skilled in the art.
Notwithstanding the advantages of the ion implantation technique a masking procedure is still required in order to produce various device configurations. Likewise, where several layers of material or metallization is desired in selective areas of the device it is often more convenient to deposit the material over the complete area a material which can be dissolved and lift-off portions of the material not desired in particular areas. Therefore, the invention pertains to that area of technology wherein semiconductor device manufacture utilizes ion implantation and the associated masking procedures and to the lift-off practices long used and well known to those skilled in the art of semiconductor device manufacture.
The use of a thin layer of material, usually 100-500 A silicon dioxide, as a screen for ion implantation is well known. The purpose of this screen material is two fold: (1) to randomize the direction of the ions incident on the silicon surface, thereby minimizing the chance of these ions penetrating deep in the material by channeling along easy open crystal directions, and (2) to prevent contamination of the silicon surface with sputtered impurities. The use of silicon dioxide as a screen material for ion implantation is very convenient and compatible with device processing. However, in the process of implanting through a silicon dioxide screen, oxygen is driven from the silicon dioxide layer into the silicon substrate by the implanted ions. This phenomena is referred to in the literature as the "knock-on" effect. Oxygen in the substrate is known to contribute to device degradation through introductions and nucleation of stacking faults.
It is generally well known that photoresist is an attractive material for use in masking an area for ion implantation. It is convenient to use; once deposited, developed, and etched the mask is complete and no further processing is required. The edges of the mask opening are sharp and well defined and undesired ions are removed when the photoresist mask is stripped. The popularity of photoresist as an ion implantation mask has been impeded since the photoresist which is usually a complex organic compound or mixture thereof, is susceptible to cross linking during ion implantation and forms substances difficult to remove from, for example, a semiconductor substrate. Removal of this material is usually accomplished by oxidation using nascent oxygen, strong acids, or other oxidizing reagents. Organic solvents are ineffective in removing cross linked photoresist.
Furthermore, during the process of ion implantation using a photoresist mask, hydrocarbons are sputtered into the mask opening and react with the silicon or silicon dioxide to form such substances as silicon carbide thereby in the case of field effect transistor manufacture causing a degradation of the gate oxide and other associated problems.